The useful lifetime of rolling contact fatigue elements such as bearings can be shortened significantly if the element is not free of surface or subsurface defects. Ceramic rolling elements, especially ball bearings, are rapidly being developed for commercial applications. Presently, because of their mechanical and physical properties, Si.sub.3 N.sub.4 ceramics are preferred for use in ceramic rolling elements as they offer a number of advantages over steel counterparts. These advantages include higher stiffness, increased operating speeds, and improved corrosion resistance and thermal stability. In addition, the primary failure mode of Si.sub.3 N.sub.4 bearings is by spallation, the least harmful mode of failure.
Ceramic bearings are typically classified into two types: (1) hybrid bearings which include a ceramic rolling element and steel races; or (2) all-ceramic bearings which have ceramic rolling elements and ceramic races. Machining operations are typically different for each type of ceramic component. Bearings are usually machined from hot pressed "blanks" which are then hot isostatically pressed (HIPped) and finally lapped to finishes on the order of &lt;0.05 .mu.m Ra. Races on the other hand are often ground with surface finishes of &lt;0.1-0.2 .mu.m Ra. These machining operations can potentially introduce surface and subsurface defects. However, subsurface defects could also be inherently present in the material and not effect machining while still having an adverse impact on useful lifetime. Non-destructive testing methods which can be reliably applied to map surface and near-subsurface defects in a production environment are therefore highly desirable.
One non-destructive characterization (NDC) method which has been extensively studied for metal surfaces (typically optical surfaces) is optical scattering. Optical scattering occurs whenever a beam of light is incident upon a surface. Fundamental geometric optics predicts that for a perfect reflector the angle of reflection will be equal to the angle of incidence. However, if the surface on which the light is incident is transparent, or semi-transparent, then a component of the beam will be internally transmitted (except at the critical angle). Certain Si.sub.3 N.sub.4 ceramics are partially transparent at selected wavelengths. FIG. 1 is a simplified schematic diagram of an incident optical beam 11 on a partially transparent material 10. A reflected beam 12, a transmitted optical beam 14, scattered light 16 and various subsurface optical scatterers 18 are present. However, many features on a scattering surface, particularly a machined ceramic surface, can cause scatter. Some of these features are shown in simplified schematic diagram form in FIG. 2, which shows a range of surface roughness, surface particulate, cutting oils and subsurface defects of different shape, orientation and location.
A non-destructive method capable of discriminating between surface defects and subsurface defects is highly desirable. One common approach employs polarized laser light incident on the surface of the material being analyzed as shown in simplified schematic diagram form in FIG. 3. In FIG. 3, "P" represents P polarized light where the electric vector is parallel to the plane of incidence and "S" represents S polarized light where the electric vector is perpendicular to the plane of incidence. As shown in FIG. 3, a portion of the incident light is transmitted through the material and a portion is reflected as well as scattered from the surface of the material. The material under study is shown in the X-Y plane. For subsurface defect detection, surface scatter can be considered noise and subsurface scatter the signal. To improve the signal-to-noise (S/N) ratio one must either remove the surface effects for subsurface defect detection, or enhance surface topography scatter by removing subsurface scatter effects.
For all transparent dielectric materials, each of the two orthogonal polarizations (s and p) have different reflectivities. In addition, these reflectivities are a function of angle of incidence. One unique feature of this functionality is that for a given material there is a single angle, known as the Brewster angle, at which the reflectivity for p-polarized light is exactly zero. This means that by illuminating a specimen at its Brewster angle, the surface or subsurface may be predominantly examined by alternating the laser beam from s- to p-polarization, respectively. This Brewster angle is defined as ##EQU1## where n.sub.i is the refractive index of the incident medium (usually air) and n.sub.t is the index of the material itself. Most ceramic materials have an index on the order of 2-3, thereby indicating a Brewster angle of 63.degree. to 72.degree..
Several detector schemes have been studied in the past. One approach incorporating a total integrated scatter (TIS) measuring apparatus 20 shown in simplified schematic diagram form in FIG. 4 typically employs a He-Ne laser 22, an optical component 24 to filter the laser beam, a hemispherical collecting mirror 26 to collect the total scattered light over 2 .pi. steradians, and first and second detectors 28 and 30 to respectively detect direct scattered light and specularly scattered light. The TIS measuring apparatus 20 has been used to study surface finish and TIS as it relates to surface finish as expressed by the following equation: ##EQU2## where V.sub.s =Voltage of detector for scattered light only
V.sub.s+.DELTA.s =Voltage of detector for scattered light PLUS specularly scattered light PA1 .delta.=Surface roughness in rms..mu.m PA1 .lambda.=Wavelength of incident light..mu.m
Another approach employing an angle-resolved scattering (ARS) measurement apparatus 32 shown in simplified schematic diagram form in FIG. 5 also includes a He-Ne laser 34. The ARS measurement apparatus 32 further includes such elements as a chopper 36, a polarizer 38 and a spatial filter 40 in the input optics to, among other things, set the polarization, and an analyzing polarizer 42 in the detector 48 which also includes a receiving telescope 44 and a lock-in amplifier 46. The detector 48 is mounted on a rotating stage to detect angular dependent scatter.
Another analytical approach to scattering distribution analysis is the Bi-direction Scatter Distribution Function (BSDF) first proposed by Nicodemus et al. in 1977. Variations of this function are BRDF, BTDF and BVDF for reflective, transmissive and volume scattering, respectively. All variations, however, are considered subsets of BSDF which is usually quantified in radiometric terms as the quotient of the scattered surface radiance divided by the incident surface radiance. Referring to FIG. 6, there is shown a schematic diagram illustrating the geometry used in defining BRDF, where the scattered surface radiance S is defined as the light flux (dp) scattered per unit surface area (A) per unit solid angle and is given as follows by Eq. 2. ##EQU3## where projected solid angle =d.OMEGA..sub.s .multidot.cos .theta..sub.s.
The incident surface irradiation is the light flux on the surface per unit of illuminated surface area and is expressed as ##EQU4##
Thus, the BRDF is given by the following ##EQU5##
Because the BRDF accounts for differences in reflected light, the BRDF will be modified as a function of the position of the analyzing polarizer and the type of scatterer.
In addition to the optical approaches to surface and subsurface defect detection and characterization described above, x-ray CT (computer tomography) scanning has also been used to detect defects in ceramic products. However, this technique suffers from limitations in detecting defects in the near-surface layer in curved or other complex shapes due to an averaging of the signal based on data from the sample and air. The image is formed from square pixels giving rise to irregular edges and limited spatial resolution. Recently, ultrasonics has been used for detecting surface and near-subsurface defects, but this approach is currently limited by coupling problems.
The present invention overcomes the aforementioned limitations of the prior art by providing a non-destructive method and apparatus for detecting surface and near-subsurface defects in dense ceramics and particularly in ceramic products with complex shapes such as ceramic bearings, turbine blades, races and the like.